This invention relates to apparatus and methods for forming coils of optical fiber. In particular, this relates to apparatus and methods for forming sensing coils for fiber optic rotation sensors. Still more particularly, this invention relates to apparatus and methods for forming quadrupole-wound fiber optic sensing coils.
It has been found that a fiber optic rotation sensor that meets the performance requirements for guidance and navigation systems should have a sensing coil having quadrupole-wound symmetry. Quadrupole-wound symmetry is attained by dividing a continuous length of optical fiber into two or more source spools and winding optical fiber from each spool onto a reel. A single layer of the optical fiber coil is first wound on the reel. Then alternating dual layers of optical fiber are wound from each source spool onto the reel. A dual layer is two layers arranged with an outer layer formed over an inner layer such that the wrapping of the outer layer ends where the inner layer begins. The optical fiber is then wound onto the reel until selected equal lengths from each source spool are placed on the reel.
The quadrupole winding pattern preserves symmetry about the center of the fiber and, therefore, decreases phase errors due to changing thermal conditions. Phase errors caused by thermal gradients in fiber optic rotation sensors are discussed by N. J. Frigo, "Compensation of Linear Sources of Non-reciprocity in Sagnac Interferometers," SPIE Proceedings, Fiber Optic and Laser Sensors, Vol. 412, pp. 288-271.
The basic structure of fiber optic coils having quadrupole-wound symmetry is explained in U.S. Pat. No. 4,856,900, which issued Aug. 15, 1989 to Ivancevic. U.S. Pat. No. 4,856,900 is assigned to Litton Systems, Inc. The disclosure of U.S. Pat. No. 4,856,900 is hereby incorporated by reference into the present disclosure.
A coil formed with quadrupole-wound symmetry has the property that lengths of the fiber which are equidistant from the center of the spool are in such close proximity that they will have essentially the same temperature. Therefore, temperature gradients are symmetrical about the center of the quadrupole-wound coil. Phase errors due to the Shupe effect are, therefore, also symmetrical such that Shupe effect phase errors on opposite sides of the center of the coil are of opposite sign and tend to negate each other.